by Douglas Robson, USA TODAY Sports

by Douglas Robson, USA TODAY Sports

WIMBLEDON, England - Serena Williams and Sloane Stephens continued their fine form at majors on Saturday, booking spots in the fourth round at Wimbledon.

Williams, 31, and Stephens, 20, represent opposite ends of the current generation of American women - and so far in 2013, they have been its most consistent players.

They are the only Yanks to reach the second week in all three Grand Slam events - and now have chances to go further when play resumes Monday.

"Both generations getting it done, and both generations are getting better," Patrick McEnroe, general manager of USTA player development, wrote in an email.

Stephens, who upset Williams to reach the Australian Open semifinals and is seeded 17th, shook off a sluggish start in a match delayed by darkness Friday to beat 196th-ranked qualifier Petra Cetkovska of the Czech Republic 7-6 (7-3), 0-6, 6-4.

"It was definitely tough," said Stephens, who blamed the waning light for her mid-match slump. "Lost focus there. But, I mean, it's a Grand Slam, so you just have to play hard, just keep going, know battling will go a long ways."

Top-seeded Williams showed no respect to her elders and dispatched 42-year-old Kimiko Date-Krumm of Japan 6-2, 6-0 in 61 minutes.

The pair started play past 8 p.m. under a the closed roof on Centre Court when previous matches on their originally scheduled Court 1 ran long.

"I don't think it gets better for me than a closed roof on grass," said Williams, who improved her career-best winning streak to 34-0 and notched her 600th WTA victory.

Williams was generally happy with the match ... that doesn't mean, however, she didn't find things she wants to do better.

"I feel like I'm moving well," she said "I feel like I'm covering the court OK. You know, I'm pretty much insatiable. There's always room that I look to improve. I would love to improve my serve, I would love to improve my return. I think my return was better today than it was in my last match."

The other two Americans on the court Saturday, Madison Keys and Alison Riske, could not join Williams and Stephens.

Riske, a wild card making her Grand Slam debut whose best results have come on grass, lost to No. 46 Kaia Kanepi of Estonia 6-2, 6-3.

Keys, 18, does not yet have the long-rally reliability or all-court movement on grass to fend off a defensive whiz like Radwanska.

But the young player with developing weapons is coming fast.

She played No. 4 Radwanska much closer than she did in March when she managed just two games on hardcourts in Miami.

"She was really playing great tennis," Radwanska said. "Especially she was serving unbelievable."

After ending last year at No. 149, Keys is projected to climbed into the mid-40s when Wimbledon concludes, making her one of six Americans in the top 50, the most since November of 2006.

"There has been a lot of improvement," Keys said.

As a group, however, grass has proved poorer footing than clay with only two of 14 still alive. At the French Open, where 15 American women started in the main draw, four reached the Round of 16 - Williams, Stephens, Jamie Hampton and Bethanie Mattek-Sands.Only eventual champion Williams advanced.

But opportunity abounds.

Stephens overcame tough tests in the first two rounds against Hampton and former top-10 player Andrea Petkovic of Germany. Now she is the second-highest seed in her quarter of the draw after No. 15 Marian Bartoli of France.

No. 5 Sara Errani and No. 9 Caroline Wozniacki were eliminated from her section during an upset-laden first week.

Stephens will have a decent shot to reach her second Grand Slam semifinal of 2013. Her first obstacle is Monica Puig of Puerto Rico.

Puig, 19, and Stephens go way back. They once trained at the same academy in Florida as juniors.

"I've known her for a really long time," she said, but noted that they were "not besties."

Williams faces her sternest test of the fortnight in fourth-round opponent Sabine Lisicki, the No. 23 seed from Germany. A 2011 Wimbledon semifinalist, the big-serving Lisicki owns a 16-4 record at the All-England Club and 16-15 at the other three majors.

On Saturday, Stephens again sidestepped questions about Williams that have dogged her since criticizing the 16-time Grand Slam champion for personal and social media slights in two national magazine articles this spring.

Asked about the possibility of squaring off against Williams in the Wimbledon final, Stephens said such projections were useless until they reach that stage of the tournament.